In a related application entitled "Error Correction Code and Write Error Preservation for Add-on Memory", a system and method is disclosed which permits add-on memory cards to contain error correction code in which data words are comprised of multiple data bytes. However, the system disclosed in this related application can function as disclosed only when an entire data word is written which includes all of the data bytes since bits from each of the data words are included in generating the check bits. In another related application entitled "Error Correction Code on Add-On Cards for Writing Portions of Data Words", a technique is provided to perform read-modify-write cycles on add-on cards in a system that does not have error correction code. This would return an error signal during initialization of the computer.